I'm compiling this on linux. It will compile and run, but when I enter values for n and p, this is what my terminal looks like:
7
1.0
Segmentation fault (core dumped)
In this case, 7 is the input for n, and 1.0 is the input for p. I've tried this with different values. The idea is to use Dynamic Programming to fill in a 2D array of probabilities through recursion. Let me know if you need more info, but this is the entirety of the code.
#include <iostream>
using namespace std;
int main(int argc, char *argv[])
{
int n;
double p;
cin >> n;
cin >> p;
cout << n;
cout << p;
cout << "Initializing array.";
double** probability = new double*[n];
for(int i = 0; i < n; ++i)
{
probability[i] = new double[n];
}
//cout << "Beginning filling i loop.";
for(int i = 0; i < n; i++)
{
probability[i][0] = 0;
}
//cout << "Beginning filling j loop.";
for(int j = 0; j < n; j++)
{
probability[0][j] = 1;
}
//cout << "Beginning filling nested loop.";
for(int i = 1; i< n; i++)
{
for(int j = 1; j< n; j++)
{
probability[i][j] = (p * probability[i-1][j]) + ((1-p) * probability[i][j-1]);
}
}
cout << "Probability: ";
cout << probability[n][n];
//cleanup
for(int i = 0; i < n; ++i)
{
delete probability[i] ;
}
delete probability;
return 0;
}
cout << probability[n][n];
probability[][] is an n by n array. The last element is probability[n-1][n-1] , so you are running off the end of the array and invoking undefined behavior.
Related
I was making a cpp program in which it takes two input from the user which determine the size of the 2d array and pass the values to the mat class constructor and dynamically create an array of the user's defined size. But, I don't know why it is not working and showing segmentation fault
#include<iostream>
using namespace std;
class mat{
int **a;
int r, c;
public:
mat(int row, int col){
r = row;
c = col;
for(int i = 0; i < r; i++){
*a = new int[r];
*a++;
}
}
void input(){
for(int i = 0; i < r; i++){
for(int j = 0; i < c; j++){
cin >> a[i][j];
}
}
}
void display(){
for(int i = 0; i < r; i++){
for(int j = 0; i < c; j++){
cout << a[i][j] << "\t";
}
cout << endl;
}
}
};
int main()
{
int r, c;
cout << "enter row :";
cin >> r;
cout << "enter column :";
cin >> c;
mat m(r, c);
m.input();
cout << "array \n";
m.display();
}
I can feel that the issue is with the for loop in the constructor or maybe I am doing it wrong.
The class contains several errors.
The variable a is never initialized. When we try to address the memory pointed to by a we get a segmentation fault. We can initialize it like this a = new int*[r]
We should not change where a point's to, so don't use a++. Otherwise a[i][j] will not refer to the i'th row and the j'th column. We would also want to release the memory at some point.
The inner loop for the columns for(int j = 0; i < c; j++) once entered will never terminate and will eventually produce a segmentation fault. We need to change i < c to j < c.
If we fix these errors, it looks like this:
class mat {
int** a;
int r, c;
public:
mat(int row, int col) {
r = row;
c = col;
a = new int*[r];
for (int i = 0; i < r; i++) {
a[i] = new int[c];
}
}
void input() {
for (int i = 0; i < r; i++) {
for (int j = 0; j < c; j++) {
cin >> a[i][j];
}
}
}
void display() {
for (int i = 0; i < r; i++) {
for (int j = 0; j < c; j++) {
cout << a[i][j] << "\t";
}
cout << endl;
}
}
};
Firstly when I have code this program it was running perfectly but running it again, it is not showing expected output can someone tell what's wrong with it
#include<bits/stdc++.h>
using namespace std;
int main(){
int n;
cin >> n;
int arr[n];
int loc,min;
for (int i = 0; i < n; i++)
{
cin >> arr[i];
}
for (int i = 0; i < n - 1;i++){
min = arr[i];
for (int j = i + 1; j < n; j++)
{
if(min>arr[j]){
min = arr[j];
loc = j;
}
swap(arr[loc],arr[i]);
}
}
for (int i = 0; i < n; i++){
cout << arr[i] << " ";
}
cout << endl;
}
Forgoing the fact that variable-length arrays are not part of standard C++ (and thus code tutorials that use them should be burned), the code has two main problems.
On an already sorted sequence, the inner-most if body will never be entered, and therefore loc will never receive a determinate value.
The swap is in the wrong place..
Explanation
Within your code...
using namespace std;
int main(){
int n;
cin >> n;
int arr[n];
int loc,min; // loc is INDETERMINATE HERE
for (int i = 0; i < n; i++)
{
cin >> arr[i];
}
for (int i = 0; i < n - 1;i++){
min = arr[i];
for (int j = i + 1; j < n; j++)
{
if(min>arr[j]){
min = arr[j];
loc = j; // loc ONLY EVER SET HERE
}
swap(arr[loc],arr[i]); // loc IS USED HERE EVEN IF NEVER SET
}
}
for (int i = 0; i < n; i++){
cout << arr[i] << " ";
}
cout << endl;
}
The purpose of the inner loop is to find the location (loc) of the most extreme value (smallest, largest, whatever you're using for your order criteria) within the remaining sequence. No swapping should be taking place in the inner loop, and the initial extreme value location (again, loc) should be the current index of the outer loop (in this case i)
Therefore...
We don't need min. It is pointless.
We must initialize loc to be i before entering the inner loop.
We swap after the inner loop, and then only if loc is no longer i.
The result looks like this.
int main()
{
int n;
cin >> n;
int arr[n];
for (int i = 0; i < n; i++)
{
cin >> arr[i];
}
for (int i = 0; i < n - 1; i++)
{
int loc = i;
for (int j = i + 1; j < n; j++)
{
if (arr[loc] > arr[j])
loc = j; // update location to new most-extreme value
}
// only need to swap if the location is no longer same as i
if (loc != i)
swap(arr[loc], arr[i]);
}
for (int i = 0; i < n; i++)
{
cout << arr[i] << " ";
}
cout << endl;
}
The line swap(arr[loc],arr[i]); should be outside the inner for loop, so move it one line down.
Also, you will want to initialize loc to i at the start of the outer for loop.
#include<bits/stdc++.h>
using namespace std;
int main(){
int n;
cin >> n;
int arr[n];
int loc,min;
for (int i = 0; i < n; i++)
{
cin >> arr[i];
}
for (int i = 0; i < n - 1;i++){
min = arr[i];
loc=i;
for (int j = i + 1; j < n; j++)
{
if(min>arr[j]){
min = arr[j];
loc = j;
}
swap(arr[i],arr[loc]);
}
}
for (int i = 0; i < n; i++){
cout << arr[i] << " ";
}
cout << endl;
}
Here is my code:
#include<iostream>
#include<cstdlib>
using namespace std;
int main() {
int** arr=NULL;
int num=0;
cin >> num;
int* big=NULL;
arr = new int*[num];
for (int i = 0; i < num; i++) {
arr[i] = new int[5];
}
big = new int[num];
for (int i = 0; i < num; i++) {
for (int j = 0; j < 5; j++) {
while (1) {
cin >> arr[i][j];
if (arr[i][j] >= 0 && arr[i][j] < 100)
break;
}
}
}
for (int i = 0; i < 5; i++) {
big[i] = 0;
}
for (int i = 0; i < num; i++) {
for (int j = 0; j < 5; j++) {
if (big[i] < arr[i][j]) {
big[i] = arr[i][j];
}
}
}
for (int i = 0; i < num; i++) {
cout << "Case #" << i + 1 << ": " << big[i] << endl;
}
delete[]big;
for (int i = num-1; i>=0; i--) {
delete[]arr[i];
}
delete[]arr;
return 0;
}
When I run this code, it says that there are heap corruption error (heap corruption detected). I think it means that there are some errors at 'new' or 'delete' parts in my codes, but I cannot find them. I hope someone to answer. Thanks.
Error is here:
big = new int[num];
...
for (int i = 0; i < 5; i++) {
big[i] = 0;
}
So when you have num less than 5 you are writing outside the array.
Anyway you are using C++ so use vector for such tasks.
#include<iostream>
#include<cstdlib>
#include<vector>
using namespace std;
int main() {
vector<vector<int>> arr;
int num=0;
cin >> num;
arr.resize(num, vector<int>(5));
for (auto &row : arr) {
for (auto &cell : row) {
while (1) {
cin >> cell ;
if (cell >= 0 && cell < 100)
break;
}
}
}
vector<int> big(arr.size());
for (int i = 0; i < arr.size(); i++) {
for (auto &cell : arr[i]) {
if (big[i] < cell) {
big[i] = cell;
}
}
}
for (int i = 0; i < num; i++) {
cout << "Case #" << i + 1 << ": " << big[i] << endl;
}
return 0;
}
In many places in your code, you're indexing your big array using indexes from 0 to 5, while the array is allocated using user input, if user input was 4 for example, your code is undefined behavior.
If you're using c++, you shouldn't be manually allocating the arrays, use std::vector instead, it will take care of managing memory for you, so you don't have to new and delete memory yourself.
With std::vector, your code would look somewhat like this.
std::vector<std::vector<int>> arr;
std::vector<int> big;
cin>>num;
arr.resize(num, std::vector<int>(5));
big.resize(5);
You will also be able to use at method to access elements while bound-checking, and size method to get the number of elements of the array.
I'm really stuck with my code for Gauss Elimination in C++, I need to return upper triangular matrix but still only thing I get is Segmentation fault. I know there must be some sort of going of allocated memory but I can't find where.
Code:
#include <iostream>
using namespace std;
double ** allocateDynamicArray(int order){
double ** dynArray = new double *[order];
int cols = order+1;
double *pool = new double [order * cols];
for(int i = 0;i < order; i++, pool += cols){
dynArray[i] = pool;
}
return dynArray;
}
void deallocateDynamicArray(double **dynArray){
delete [] dynArray[0];
delete [] dynArray;
}
void addAndPrintArray(double **dynArray, int order){
cout << "Zadejte prvky pole radu " << order << endl;
for(int i=0; i< order; i++){
for(int j=0; j< order+1; j++){
cout << "Pole[" << i << "][" << j << "]: ";
cin >> dynArray[i][j];
}
}
for(int i=0; i< order; i++){
for(int j=0; j< order+1; j++){
if(dynArray[i][j] < 10 && dynArray[i][j] >= 0){
cout << " ";
}
cout << dynArray[i][j] << " ";
}
cout << endl;
}
}
double ** gaussElimination(double** dynArray, int order){
for(int j=1; j<=order; j++) /*Horni trojuhelnikova matice*/
{
for(int i=1; i<=order; i++)
{
if(i>j)
{
double c=dynArray[i][j]/dynArray[j][j];
for(int k=1; k<=order+1; k++)
{
dynArray[i][k] = dynArray[i][k] - c * dynArray[j][k];
}
}
}
}
return dynArray;
}
int main()
{
cout << "Zadejte rad matice: ";
int order;
cin >> order;
double **arr = allocateDynamicArray(order);
addAndPrintArray(arr, order);
gaussElimination(arr, order);
deallocateDynamicArray(arr);
return 0;
}
Can anyone tell me what's wrong?
The problem is that in C/C++ the first element of an array should have index 0, so your
for(int i=1; i<=order; i++)
should be
for(int i=0; i<order; i++)
in the gaussElimination function.
I'm trying to create a 3D char array with dynamic memory. I create the char*** point in main then pass it to input and everything works fine until the input function returns and i try to repring the same locaton from main. I get "Access violation reading location." Any suggestions?
void input(char ***a, int f, int n)
{
cin >> f;
cin >> n;
a = new char**[f];
for (int i =0; i <f; ++i)
{
a[i]= new char*[n];
for (int j=0; j <n; ++j)
{
a[i][j] = new char[n];
}
}
for (int i =0; i<f; ++i)
{
for (int j=0; j<n; ++j)
{
for (int k = 0; k <n ;++k)
{
a[i][j][k] = '.';
cout << a[i][j][k];}
}
}
cout <<endl << endl<< a[2][5][5]; //test to see is value is '."
}
int main()
{
char ***station = 0;
int floors=0, n=0;
input(station, floors, n);
cout << endl << endl << station[2][5][5];
}